Sahih Bukhari Volume 5, Book 58, Hadith Number 249.

Narated By ‘Aisha : The first child who was born in the Islamic Land (i.e. Medina) amongst the Emigrants, was ‘Abdullah bin Az-Zubair. They brought him to the Prophet. The Prophet took a date, and after chewing it, put its juice in his mouth. So the first thing that went into the child’s stomach, was the saliva of the Prophet.